Occurrence of powdery mildew (Erysiphe sp.) on Echeveria spp., Crassula spp., Cotyledon and Dudleya in the UK

The Crassulaceae is a large family including 1400 species in 33 genera. They are popular horticultural plants grown for their spectacular flower heads and unique leaves, which vary in shape, size and texture. In the genus Cotyledon, plants are also used medicinally for the treatment of warts and abscesses. In February 2007, an outbreak of powdery mildew occurred on a large collection of Crassulaceae in the glasshouse at Wisley gardens. The plants affected included Echeveria gibbiflora, E. gibbiflora 'Carunculata', E. 'Chantilly', E. 'Afterglow', E. 'Secunda', E. 'Ballerina', Crassula capitella 'Flame', C. lactea, C. multicava, Dudleya palmeri and Cotyledon orbiculata (Fig. 1).

The mycelium was amphigenous causing corky lesions on both stems and leaves. Morphological characteristics of the fungi isolated from all plants were similar. The conidia were cylindrical, lacking fibrosin bodies, length 27 - 47 µm (mean 34 µm), width 10 - 18 µm (mean 14 µm). Conidiophores were erect consisting of a straight foot-cell 15 - 38 µm (mean 28 µm) x 7 - 11 μm (mean 9 μm) followed by 2 distal cells (Fig.2). Conidia were formed singly. Appressoria on the mycelium were lobed to multilobed. No chasmothecia were present but the above characteristics are consistent with Oidium subgenus Pseudoidium, the anamorph of Erysiphe, and with the powdery mildew found on Sedum alboroseum in Hungary (Jankovics and Szentiványi, 2006).

The ITS regions of isolates from E. gibbiflora, E. gibbiflora 'Carunculata, E. 'Chantilly', C. capitella 'Flame', D. palmeri, Cotyledon orbiculata and the Hungarian isolate found on Sedum were amplified and sequenced using the primers PMITS1 and PMITS2 (Denton and Henricot, 2006). All isolates had identical ITS sequences (GenBank Acc. No EF434394, EU185636 to EU185641) and did not match fully any powdery mildew species published in GenBank. Records of Podosphaera (syn. Sphaerotheca macularis) on Crassula (Farr et al., 2007), Golovinomyces (syn. Erysiphe) orontii on Crassula, Echeveria and Cotyledon (Braun, 1987) and Neoerysiphe galeopsidis (Kiss, 1999) on Crassula have been found. The DNA and the morphology rule out these species but confirm our species is the same as the one affecting Sedum in Hungary. This is the first record of this powdery mildew species on Crassula, Echeveria, Cotyledon and Dudleya in the UK.